The southern end of the Strawhouse trail begins near the Boquillas Canyon
overlook, from which you can get the following view of the town of Boquillas
del Carmen, perched on a
fluvial fill terrace
of the Rio Grande, consisting of water-deposited
alluvium, deposited by the
river during flood stages when its bed was higher. (Since this feature and
others discussed later are in Mexico, I am extrapolating from what was mapped
on the U.S. side, but I think what I have here is correct.) Subsequently, the
alluvium has been eroded to create terrace isles like this one. For years since
the attack of September 11, 2001, fear-mongering politicians and government
officials have virtually cut the village off from the outside world by
eliminating crossings into the US. Access to
needed supplies was extremely difficult through Mexico, a situation I believe
continues today. Finally, the US government has established a controlled
crossing, a move that has brought foam-flecked outrage from extreme right
wingers about floods of illegal aliens spreading like fire ants across Big Bend
and has done not much, as far as I can tell, to really help the citizens of
Boquillas del Carmen return to the lives they used to know. You have to go
through official scrutiny both ways by officials of both governments - not the
informal crossing it used to be. At least it's not as bad as the TSA, and maybe
a modicum of tourism will help the town get back on its feet.

The trailhead is posted as the Marufo Vega Trail but is also the southern end
of the Strawhouse trail. Farther to the north the two trails separate, with the
Marufo Vega heading east and the Strawhouse continuing north. As you find when
hiking to and into Telephone Canyon,
the terrain along the trail as you head north is due largely to the faulting
associated with the
Basin and Range
geologic province of the western US, which has produced
fault-block
mountains on the eastern side of Big Bend National Park. Relative tension
in the Earth's crust beginning around 20 million years ago in much of the
western US and parts of Mexico led to the
crustal
extension responsible for this type of mountains. In the image below, taken
early in the hike, you see a fault scarp of Basin and Range age, which has
uplifted the Santa Elena
Limestone of lower
Cretaceous age on the far
side of the fault, and
Miocene deposits that filled
in low areas (basins) between fault-block mountains. (The Miocene occurred from
23 to 5 million years ago.) The material you are hiking on (when you are out of
the wash) is alluvial, deposited in the middle to late
Pleistocene, roughly
2 million to 12 thousand years ago when the climate turned cooler and wetter
with every new glacier advance into what is now the US.

The next picture is a close-up of the Miocene basin-fill deposits. They
consist of sand, gravel, and cobbles. Note the cross-bedding and how
well-rounded the
clasts are. (A clast is a piece of geological detritus, ranging in size
from clay to boulder.) This indicates the material was deposited in a stream
environment. The figure after that is a diagram of cross-bedding. The
basic process consists of deposition, erosion, and more deposition, often with
changing current directions and changing erosional slopes on which new
deposition occurs. Referring to the arrows of flow direction in the second
figure, you can see that the lower bed in the image was deposited by water
flowing generally from right to left. (This was well before the advent of the
Rio Grande. Now the flow is in the opposite direction.) Also note the
caliche layer at the surface,
which forms a hard layer protecting the material beneath it from erosion.

Shortly after the above picture was taken, I came across ruins of the old
ore tramway that used to operate here. It carried lead, zinc, and silver ore
from the Puerto Rico mine in Mexico four miles to a terminal where mule trains
took the ore north to Marathon and the rail line. More on this tramway can be
found here. The picture that
introduces this virtual field trip is of the ruins I came across. You can right
click on the picture to see it enlarged. More of the Miocene basin-fill
deposits are visible in the background.

If you're lucky, you will eventually come upon a sign in the wash that marks
the split between two legs of the Strawhouse Trail. The left trail goes through
a canyon where you have to do a lot of boulder hopping - not a route to take on
horseback. The right trail goes up over a small saddle to a point where it
splits again - into the continuation of the Strawhouse Trail on the left that
descends back into the wash and
the Marufo Vega Trail on the right. I met a couple who had missed
the sign and were heading the wrong way, into the canyon instead of up through
the saddle to the Marufo Vega Trail, which was the trail they wanted. However,
they were clearly not prepared for a hike in the desert, so it's just as well
they decided to turn back. (The woman was wearing very short shorts, for
example. As one person has said, Big Bend is full of vegetables with knives.)
The point is, the park service needs to mark the place where the trail divides
much more clearly. I didn't see the sign on my way back, though I was looking
for it.

I decided to take the right trail to the saddle, which crosses the fault
responsible for the fault scarp pictured earlier. The fault is covered with
rock debris here, but you can see its effects in the next picture. Note the
rocks on the left (Santa Elena Limestone) are steeply
dipping left, to the
east, whereas those on the right are dipping much less.

The trail up to the saddle follows a spectacular and large
vein of
calcite, as depicted in the
following two images. It must have stretched for around 100 feet (about 30
meters) along the trail, and some of the crystals displayed are quite large.
The Santa Elena, the rock here, is a limestone, which has abundant calcium
carbonate ("Tums" ;-) in the form of the mineral calcite. Calcite is relatively
easily dissolved in water, especially acidic water, and dissolves more readily
when subject to pressure. A likely explanation of the occurence of this vein is
that the pressure associated with the faulting produced fractures which water
pressure could force open. The water contained dissolved calcium carbonate,
which crystallized into calcite. Calcium carbonate, due to its abundance, is
responsible for many calcite veins filling cracks under a variety of geological
conditions. The fact the crystals in these pictures are perpendicular to the
sides of the fracture indicates no lateral movement has occurred along it.

When you cross to the other side of the saddle, you see that the beds are
no longer dipping to the west, but rather to the east, as seen below. Note
that the rocks are pretty broken up on both sides of the fault.

Fossils can be found in detrital rocks along the path. In the following
picture you can see mostly clams and/or oysters. The distortions these fossils
have undergone in the fault zone is clearly visible in the shape of the
gastropod (snail) near
the center bottom of the picture. As always, I like to point out that
it is illegal - not to mention selfish - to collect samples from a national
park. Park rangers have told me that people often attempt to get away with it,
however. That's pretty sad.

After examining the geology on the saddle, I returned to the wash in order
to go through the canyon the saddle trail avoids. The entrance to the canyon is
shown below. The fault goes through here, perpendicular to the trend of the
canyon, and the rocks certainly appear to have been subject to a lot of
trauma.

As you make your way north through the canyon you see rocks dipping similar
to those on the saddle, that is, in a generally westerly direction. The
following picture is looking to the northeast. The degree of the slope here is
determined by the dip of the beds. When beds dip parallel to a slope, there is
enhanced danger of rock slides, something engineers have to take into account
when construction involves slope-forming work. The dipping beds are likely on
the western side of the Bend
monocline, an extensive
monocline in this area of the park. A monocline is a sort of "kink" in the
rocks, where rock strata that are horizontal trend into dipping strata then
back to horizontal again. Monoclines are, to my knowledge, always (or almost
always) evidence of subsurface faults. The "kink" locates the position of the
fault, where the rocks of the monocline have been shifted down on one side
relative to the other without being fractured. In other words the fault does
not extend to the surface. The kink in this monocline is farther to the
east.

After exiting the canyon and much scrambling over large boulders, you
begin to see the Del Rio Clay, first in the wash and then in outcrops on the
side of the wash. The Del Rio Clay rests
stratigraphically on
the Santa Elena Limestone and is upper Cretaceous in age. It is a
shale, that is, rock made of
mud that has rock
cleavage, which
gives it its layered appearance. These layers are typically thin and are due
to flaky clay minerals that have been deposited in quiet water such that they
are oriented parallel to the bedding. There has to be a source of sediment for
shale to be deposited, so evidently there was a change in the depositional
environment between the Santa Elena - a limestone deposited in a shallow
tropical sea (Texas was in the tropics at this time) - and the Del Rio, probably
deposited when the area became a shallow and quiet region of the continental
shelf, but with a source of fine sediment from the continent. The picture of the
outcrop below was taken looking north.

In the next picture the Del Rio Clay on the right has been eroded by water
carrying the sediment of the Miocene basin-fill deposits on the left, creating
what is known in geology as an
angular unconformity.
An unconformity marks a break in deposition in the geologic record. An angular
unconformity separates beds dipping at one angle from those dipping at a
different angle, and typically indicates a large time gap in the geologic
record. That is true here, where the missing record must be on the order of
100 million years.

As you continue up the wash that serves as the Strawhouse Trail, you come
across upper Cretaceous rock strata that were deposited later than the Del Rio
Clay. In the following photo, the Ernst member of the Boquillas Formation is in
the foreground with the Santa Elena Limestone in the distance. Since the
Boquillas is younger than the Santa Elena, there is obviously a fault line on
the other side of the Boquillas across which the Santa Elena has been raised.
Not seen at this location is the Buda Limestone, which lies above the Del Rio
and beneath the Boquillas. The Boquillas Formation in general is marked by
rather thin beds of rather "dirty" limestone. The Ernst member is very similar
to the San Vincente member of that formation (not seen here) that lies above
it. Like the Del Rio, it is of late Cretaceous age and was deposited in a
marine environment where there was deposition of both calcium carbonate (mostly
due to organisms using it for shells) and clay.

Before too long you approach another unnamed (as far as I know) canyon.
This canyon is of geological interest because, as in the case of
Dog Canyon, you can hike through the
guts of an old thrust fault. The thrust at Dog Canyon is the Santiago Mountains
thrust, which is of
Laramide age. There
were two major thrust-fault episodes in Big Bend. This thrust could not be
of the earlier event, the
Ouachita orogeny,
a mountain building event in the late
Paleozoic, because the
fault cuts through rocks of Cretaceous age which are much younger. Hence, I'm
assuming this thrust occurred during the Laramide mountain-building event.
The following picture is what you see when approaching this canyon. You can
make out the strongly folded rocks in the sides of the mountain in the
distance.

Zooming in for a closer look, you can see how the strata of the Santa Elena
Formation were deformed as drag folds by the thrust fault, which in relative
terms shoved rock from left to right beneath the deformed beds in this view.

This area contains complex
faulting. There are
thrust faults, normal faults, and strike-slip faults. I could have spent a lot
more time here than I had allotted, and I'd like to return some day. A normal
fault is responsible for what you see below. A normal fault is produced by
relative tension in the crust, with one side of the fault relatively up
compared to the other. A great exposed example of a normal fault is found in
Boquillas Canyon. There is no
indication of which side is up on the new USGS Scientific Investigations Map
3142, 2011, for this area of the fault, but it certainly looks like the
Santa Elena Limestone on the left (northeast) is up compared to the Boquillas
Formation on the southwest side of the fault. The weakness of rocks disturbed
in the fault zone appears to be responsible for the valley here.

Here you are, inside the canyon with vertical Santa Elena rocks on the east
side of the drainage through the canyon. These have been bent up by the
thrusting action. The trace of the thrust fault itself runs from near the
southern entrance to the canyon to the northeast along the southwest side of
the canyon ridge, according to the USGS map. I didn't find the fault trace,
which is supposed to be exposed here, as the time allotted for this part of my
trip to the park was running out.

The final picture of the trip. From this vantage point you can see the fill
terraces of the Rio Grande, like the one on which the town of Boquillas del
Carmen resides. These terraces consist of sediment deposited during the
Pleistocene, when at times the climate was wetter than it is today. There is
actually a sort of "stair-step" of terraces, with the higher ones being older.
(See the Maxwell Scenic Highway trip.)
The terraces are former flood plains. As the river continues to
erode down into its bed, especially during drier periods when the river is not
as wide, the flood plains are exposed to erosion, creating the terraces.